Clinical and laboratory studies are conducted to determine the etiological agents and neurological manifestations of acquired immunodeficiency syndrome (AIDS) in man and simian acquired immunodeficiency syndrome (SAIDS) in nonhuman primates. Patients with neurological complications of AIDS are admitted to the Neurology Service of the NIH Clinical Center. Patients admitted to other Institutes are seen by the Infectious Diseases Branch Consultation Service. Patients are evaluated to determine the spectrum of neurological illnesses found in AIDS. Appropriate virological and immunological studies are conducted by IDB and collaborating laboratories. Studies of nonhuman primates with SAIDS are being conducted simultaneously with our AIDS protocols. Clinical and immunological parameters of SAIDS are being evaluated. Findings are compared to those seen in AIDS patients. Transmission of SAIDS to other primates using tissues from SAIDS monkeys are in progress. Attempts to isolate the etiological agent(s) are being performed.